1. Field of the Invention
The present invention relates to a copying apparatus and particularly to a copying apparatus having a so-called editing function, namely, a function of forming an image of only a specified region of a document on a sheet of copy paper.
2. Description of the Prior Art
Among conventional image forming devices used in copying apparatuses or the like, which generally form an electrostatic latent image on a photoconductor by applying exposure and scanning processes to a document and develop the image by using toner, thereby to transfer it onto a sheet of copy paper, there have been proposed or provided image forming devices having a so-called editing function, namely, the function of forming an image of only a specified region of a document on copy paper.
In principle, such an editing function is performed to remove, by exposure, an electric charge on a region of a photoconductor corresponding to an undesired region of a document prior to development of the electrostatic latent image, causing it to be impossible to apply development by toner to the above stated region, whereby formation of an image of the undesired region can be prevented.
The removal of an electric charge on the above stated region is generally performed by an erasure array having a large number of light emitting elements.
Copying apparatuses having such editing function are disclosed, for example, in the following documents.
The U.S. Pat. No. 4,582,417 discloses an apparatus for forming images in which an unnecessary portion of an image of a document is erased by controlling the turning on and off of a blank lamp; U.S. Pat. No. 4,256,400 discloses a copying machine in which an unnecessary portion of an image of a document is erased by partially intercepting through use of a masking member, the light applied from a lamp to a photosensitive medium to erase a latent image; U.S. Pat. No. 4,215,929 discloses an image forming method and apparatus in which an unnecessary portion of an image of a document is erased by partially intercepting, by a masking member or polarizing plates, the light applied from a lamp to a photosensitive body to erase a latent image; and the U.S. Pat. No. 4,653,899 discloses an image forming apparatus in which an unnecessary portion of an image of a document is erased by controlling the turning on and off of an erasure array. In addition, the U.S. Pat. No. 4,543,643 discloses fundamental operation and control of a copying apparatus.
The above stated conventional copying apparatuses have, however, a disadvantage that image formation in end portions of the edited region is effected inaccurately.
For example, as shown in FIG. 1, if an image forming operation is performed to cause a region surrounded by the lines A, B, C and D (a region surrounded by the solid lines) to be a blank portion (in an erase mode), the image is formed in reality with a region surrounded by the lines A', B', C' and D' (a hatched region surrounded by the broken lines) being a blank portion.
Conversely, as shown in FIG. 2, if image forming operation is performed to cause the portion outside the region surrounded by the lines A, B, C and D (the portion outside the region surrounded by the solid lines) to be a blank portion (in a trimming mode), the image is formed in reality with a portion outside a region surrounded by the lines A", B", C" and D" (namely, the hatched portion outside the region surrounded by the broken lines) being a blank portion.
In addition, as shown in FIG. 3, if image forming operation is performed to combine the images formed by the operations shown in FIGS. 1 and 2, the hatched portion remains as a blank portion, and thus, a white frame is formed in boundary portions of the two images.
Such problem is caused by the fact that light from the light emitting elements of an erasure array has a certain peripheral extent.
More specifically, as shown in FIG. 1, if a blank portion is to be formed in the region surrounded by the lines A, B, C and D, light emitting elements located between the lines C and D are selected as the light emitting elements to be illuminated and control operation is performed so that the time of start of illumination of the selected light emitting elements and the time of end thereof coincide with the time corresponding to A and the time corresponding to B, respectively. A reference point for selection of the above stated light emitting elements and the time of start and the time of end of illumination (reference point for coincidence with the solid lines A, B, C and D) is located at the center of the light emitting surface or the irradiated light of the light emitting elements.
As a result, as shown for example in FIG. 4, a sum of an extending amount l1 from the center E of the light emitting surface L and a diffusing amount l2 of light is unavoidably generated as a "deviation amount" from the reference point at the center, and the light having the deviation amount is applied to the photoconductor, resulting in a deviation as shown by the portion defined by the outer lines A', B', C', D', and the inner lines A", B", C" D".